The present invention is directed to a method for manufacturing a plastic part which has a shape memory, which part can be restored when subjected to energy. A plastic part manufactured in accordance to the method and to the apparatus used in the method for manufacturing the part.
In the U.S. Pat. Nos. 3,455,336, whose disclosure is incorporated by reference thereto and which was the basis of German Letters Patent 1,525,815, an oblong, potentially branched tube-shaped or hose-shaped envelope of thermoplastic synthetic material was disclosed and this envelope was provided with a shape memory. Given application of energy, the envelope would be restored until, if possible, it reaches its original shape. This envelope is manufactured in a basic shape in accordance with one of the known methods, for example, by extrusion. Subsequently, it is subjected to a cross linking method, for example, by high-energy irradiation, and then is forceably brought into a second shape, for example, by widening in order to lend the shape memory. The shape is then "frozen in" in this condition. As needed, the shape memory is activated by the application of energy, for example, heat, so that the shape part strives to return to its original shape insofar as possible. Given such shaped parts, an especially high strength is definitely desirable, at least in certain regions, but due to this unstable shape, the region is not often able to have the strength. For these reasons, attempts have been made to lend these regions the required strength by additional shape-stable reinforcements as disclosed, for example, in U.S. Pat. No. 3,542,077, which was the basis of German 1 925 739 and whose disclosure is incorporated by reference thereto. In a shrinkable longitudinally divided envelope, a non-deformable reinforcements are embedded into the wall along the longitudinal side of the envelope and these reinforcements lend the end region of this envelope the required strength and shape stability in the closing regions at the long side. Given such shaped parts, however, various method steps, some of which are involved, for example, electron irradiation of the shaped part, are necessary so that additional measures for achieving the desired properties are required. In addition, the increased problems regarding the shape stability of the closure occurs in the critical region such as, for example, in the closing regions of an envelope because the undefined deformation of the closure elements cannot be excluded due to the heating.
German OS 28 56 580 also discloses a manufacturing method for technical parts having load-suitable structures of thermal plastic polymeric materials. Shaped parts of thermal plastic materials can be provided with a higher strength with the assistance of this measure which is disclosed therein as "pressure stretching" than could be previously achieved with injection molding, extruding, casting or other manufacturing methods. This method is a matter of a procedure wherein anisotropic structures are intentionally manufactured within the shaped part and these are favorable for later load cases. However, appropriate fixing processes are also required for the thermal shape stability of such stretched parts, and these fixing processes guarantee that the molecular orientation forceably introduced for the corresponding applications are preserved in the appertaining temperature range. The shape stability is thus also guaranteed in this temperature range. In this injection molding-pressing-stretching method, hereafter referred as an SPR method, molecular orientations are introduced into a preform under pressure during cooling and these molecular orientations are fixed under pressure at a definite fixing temperature. This results in the fact that after the fixing time, the condition comprising the property introduced into it, for example, higher strengths in defined regions. When, however, the article is again heated roughly to or, respectively, beyond the fixing temperature, the molecular orientations are cancelled and the article looses the properties which were forcefully introduced with the SPR method. It thus follows therefrom that such shaped parts having these new properties are only employable below the fixing temperature. This method was, therefore, applied in plastic parts where material properties regarding strength, hardness, friction behavior or resistance to chemicals as well were to be improved. In all of these improvements, however, the temperature range or, respectively, the maximum temperature applied therefor must be observed, since the properties would otherwise be lost, as already mentioned.